vagrantfile: Workaround for the atlas.hashicorp.com redirect removal.

This workaround is only for new users. Users who are/were using old
versions of Vagrant has to manually change the value of box
metadata_url. See hashicorp/vagrant#9442

Vagrantfile

@@ -19,6 +19,14 @@ if Vagrant::VERSION == "1.8.7" then
     end
 end
 
+# Workaround: Vagrant removed the atlas.hashicorp.com to
+# vagrantcloud.com redirect in February 2018. The value of
+# DEFAULT_SERVER_URL in Vagrant versions less than 1.9.3 is
+# atlas.hashicorp.com, which means that removal broke the fetching and
+# updating of boxes (since the old URL doesn't work).  See
+# https://github.com/hashicorp/vagrant/issues/9442
+Vagrant::DEFAULT_SERVER_URL.replace('https://vagrantcloud.com')
+
 Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
 
   # For LXC. VirtualBox hosts use a different box, described below.